Material that is not affected by the color reflected from other objects?
This is partially answered here:
How can I avoid colored light reflections?

this will get rid of indirect lighting, and your cloud won't reflect the ground, but neither will anything else in your scene.

Another option would be to use a different render layer for your cloud, that way you can keep the indirect lighting and your cloud will still be full white.

Finally you could use an emissive material, and then fill in shadows with a world normal comparison since you are going for the flat look. Or if you prefer a silhouette look just use an emissive.



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